The radio frequency spectrum has been used in the past for emergency rescue operations, most notably in shipping and aircraft rescues. More recently, geo-location devices have been used to track endangered species. As mobile telephony has developed, the ability to signal for help in emergency situations has also developed. However, in all of the above cases, the geo-location systems either require two-way communications, require relatively long time intervals to acquire the data needed for geo-location, or lack continuous coverage capability. In addition, the emergency transmit/receive units required are relatively large and not convenient to "wear" or conceal, and the battery systems and modes of operation require frequent battery recharging. Also inhibiting continuous use for emergency situations are the problems associated with incapacitation, such as the dialing sequence of operations necessary on current mobile phone systems.
There are a number of geo-location techniques that have been described in the past that appear in this specification. Prior art in emergency rescue at sea and for tracking of endangered species includes a number of existing satellite and ground systems dating back to World War II. These include position determination systems used in navigation, such as LORAN (a form of triangulation), Direction Finding (DF), Transit (Doppler shifts) and GPS (another form of triangulation), as well as emergency beacon location systems such as SARSAT (Doppler shifts) and Argos (Doppler shifts). A brief description of these techniques and the weaknesses relative to the proposed invention is provided in this section, as prior art background.
One of the earliest techniques, direction-finding (DF), was developed during World War II and evolved into systems in use today, such as LORAN and associated techniques using time/phase/angular difference of arrival, to determine angular direction and errors. DF techniques are suitable for use with ground-based cellular receiver networks, such as the current mobile telephone grids, but will not provide sufficient accuracy when used in conjunction with global capability satellite networks that will soon be available. This is fundamentally due to the great distances the satellites are from the transmitters (typically &gt;700 km), and the impact created by this multiplicative factor that transforms the angular uncertainty of the measurement into a position error. However, DF may be used in conjunction with this invention for certain applications where rapid proximity to the transmitter is possible.
Another approach, developed over the last few decades, utilizes Doppler shifts in the frequency of received signals to determine the location of the transmitter. It is well known in physics text books that the shift in frequency depends on the factor (v/c) cos(.theta.), where the cosine term is the angle between the velocity vector of the satellite and the satellite-beacon line-of-site vector, and c is the speed of light. NASA has employed this technique for it's SARSAT program (search and rescue satellite). One of the drawbacks of that system is that it takes several satellite passes to determine location (it determines the zero-Doppler shift direction which provides a direction perpendicular to the satellite orbit), which means that accurate locations cannot be determined for the order of hours, the time it takes for multiple satellite passes. Newer approaches (such as the Orbcomm system, which will be fully operational in two years) can determine position to the order of a few hundred meters, but takes a complete pass (of the order of 10-15 minutes) of a single satellite to acquire sufficient data. In contrast, the proposed system will only need a simultaneous single data reception by a few visible satellites to determine location. This will minimize the energy required to provide a sufficient operational lifetime for the invention.
Time difference of arrival (TDOA) and time of arrival (TOA) techniques can be considered a subset of triangulation and direction-finding, but require a minimum of three simultaneous measurements to determine position unambiguously (by the intersection of the transit-time "circles" (TOA) or the intersection of hyperbolas from pairs of TDOA relative measurements). However, TDOA/TOA in combination with Doppler can reduce this constraint, provided that clock and/or relative times of arrival uncertainties can be reduced to provide the needed accuracy. This synergism can be used in the proposed invention to reduce the number of satellites that must receive the beacon signal.
The Global Positioning System (GPS), which was developed and launched by DoD and contractor Rockwell, depends on receiver units being able to lock-on to signals from three or more GPS satellites to calculate a position. As opposed to the system proposed herein, GPS transmitters are in space, and the receiver is carried by the user; thus it is a "receive-only" system. The position errors, depending on the code the receiver is authorized to receive, are extremely small (of the order of less than a meter for the military codes). However, the units are relatively expensive ($200-$500), and in certain instances may not be able to lock-on to these signals due to terrain blockages and satellite radiated power limitations.
Other geo-location techniques include use of the amplitudes of received signals to determine location (analogous to TOA) and inertial guidance. Amplitude techniques suffer from amplitude variations induced by weather, absorption and fading due to phase interference. Advances in inertial guidance systems and miniaturization of associated components designed to measure accelerations, made principally by the Department of Defense (DoD) funded programs, will enable accurate geo-location to be made. However the systems are currently extremely expensive ($20K-$80K for avionics-quality miniaturized gyros/accelerometers) and would require frequent reference updates (of the order of hours to days) depending on the accuracy errors tolerable.
The following references are provided as examples of prior art (all refer to systems that are at least twenty years old since conception and description in the open literature), and no claim to novelty is made for these techniques, individually, as they are described in the literature:
Transit--Johns Hopkins APL TECHNICAL DIGEST January-March 1981, Volume 2, Number 1. PA1 LORAN--"LORAN Long Range Navigation", J. A. Pierce, A. A. McKenzie and R. H. Woodward, McGraw-Hill Book Company, New York, 1948. PA1 GPS--"Global Positioning System", Vol #1 NAVIGATION, The Institute of Navigation, 815 Fifteenth Street, Suite 832, Washington, D.C. 20005, 1980: PA1 Argos--"Proceedings For The Eleventh Annual Gulf Of Mexico Information Transfer Meeting", New Orleans, La., pg. 230-232, November 1990, U.S. Dept. of Interior Minerals and Management Service, Published October 1991, Contract #1435-0001-30499, MMS-91-0040, 524 pages. PA1 SARSAT--"Ambiguity Resolution For Satellite Doppler Positioning System", P. Argentiero and J. W. Marini, IEEE Transactions On Aerospace And Electronics, pg 439, May 1979. PA1 DF--"Principles Of High-Resolution Radar", August W. Rihaczek, Peninsula Publishing, P.O. Box 867, Los Altos, Calif. 94023,1985.
"Principle of Operation of NAVSTAR and System Characteristics", R. J. Milliken and C. J. Zoller
It should be noted that the geo-location determination in all of these systems is conceptually simple, relying on the intersection of solid geometry surfaces, a technique which can be found in many high school and college freshman-level text books.
The invention described herein is different in several respects from prior art described above. First, the geo-location techniques utilized are novel and different from those currently in use. All present systems, with the exception of single-pass Doppler, are "receive-only", versus "transmit-only" for this invention. The device takes advantage of the new geo-location techniques through the use of micro-miniaturization and utilizes no power until activated by one of several sequences or methods, and thus requires little or no power until used, making miniature battery power sources feasible. Since it broadcasts infrequently when activated, the energy drain on batteries allows operation over sufficient periods of time to allow rescues to be made. Further, miniaturization is made possible by compact, omni-directional antenna designs, or easily configurable antenna placements, depending on the operational frequency of the unit. The principles used in these novel techniques have also been applied to the use of beacons which emit visible or infra-red (IR) radiation.
Finally, the use of microprocessor technologies allows the sequence of operations to be activated, allowing micro-miniaturization for concealment to be possible, as well as affordable. The use of this device with satellite constellations allows global coverage, if desirable. Other units can function with cellular or wireless networks for metropolitan area coverage.